In 1952, he began playing Chester, the unofficial deputy to Marshal Matt Dillon on Gunsmoke, eventually ad-libbing the character's full name, "Chester Wesley Proudfoot". (The character's name was changed to "Chester Goode" in the television series, which featured an entirely different cast.) Baer's portrayal of Chester was generally considered his finest and most memorable role and, as he often said, the one he found most fulfilling. Baer worked on several other radio shows produced by Norman MacDonnell, including the situation comedyThe Harold Peary Show (aka Honest Harold) as Pete the Marshal, Rogers of the Gazette (loosely based on the early life of Will Rogers) as Doc Clemens, Fort Laramie, and The Adventures of Philip Marlowe.

In 1964, Baer was cast as a sheriff in an episode of Mickey Rooney's short-lived Mickey sitcom, and as a scientist in an Outer Limits episode, "Behold, Eck!" He was seen in four episodes of Hogan's Heroes and eight episodes of Bewitched in various roles as advertising clients of McMann and Tate, including one Christmas episode entitled "Sisters at Heart" that aired on ABC on December 24, 1970 in which he played the role of Mr. Brockway, the owner of a toy manufacturing firm.[5][6]

Baer voiced Ernie Keebler in the cookie commercials[1] before he suffered a stroke in 1997 which affected both speech and movement. He recovered sufficiently to make a handful of appearances at old-time radio conventions in his later years.

In perhaps his greatest theatrical performance, Baer was especially proud of his brief appearance in a little-known film, White Dog, a powerful story about racism. Baer plays a character seen at first as a kindly grandfather only to reveal himself as a hateful bigot who has trained the title character to attack black skin. Baer remarked, "Often racism, like true evil, presents itself with a smile and a handshake".

In 1969, Baer gave the eulogy at the funeral of The Andy Griffith Show castmate Howard McNear. McNear had portrayed Mayberry's Floyd the Barber and Baer had played Mayor Roy Stoner. McNear also portrayed Doc Adams in the radio version of Gunsmoke, often interacting with Baer's character, Chester Proudfoot.